To illustrate the correlation difference and similarity, we depict in Fig. 1 the correlation of 150,000 CSI samples of \({{\bf {H}}_{\rm {R}}}\)  and \({{\bf {H}}_{\rm {I}}}\)  generated from the COST 2100 indoor channel model [1]}. From the figure, there is a strong correlation in the angular domain whereas a weak correlation exists in the delay domain. In addition, correlation similarity exists between the real and imaginary parts of the complex-valued CSI matrix in the angular domain and similar phenomenon can be observed in the delay domain.
